The present invention relates to the field of medical imaging and non-destructive evaluation, and more particularly to an improved method and circuit for more accurately digitizing waveforms having a large dynamic range.
Many methods used in the fields of medical imaging and non-destructive evaluation (NDE) of materials, such as ultrasonic pulse-echo inspection, produce broad band signals with a large dynamic range. With the advent of waveform processing the ability to obtain full waveform data has become important. Back-scattered acoustic signal used in medical imaging and ultrasonic NDE are generally broad band signals with a large dynamic range. For example, such signals may fluctuate between 10 v and 0.01 v.
Conventional ultrasonic NDE methods such as viewing A-SCANs and recording C-SCANs do not require the digitization of the complete acoustic waveform. However, a significant amount of useful information may be lost by recording data corresponding to only peak amplitudes. In order to take advantage of advanced signal processing algorithms for flaw detection, such as SAFT, tomography, split-spectrum and polarity thresholding, the ability to acquire full waveform data with the highest fidelity is necessary.
Full waveform digitization of signals having a large dynamic range requires a high sampling rate and large word length. The combination of these two requirements precludes most conventional and reasonably priced A/D converters from being used to digitize full waveforms with low quantization error. For example, a common signal used for NDE of metals has a bandwidth of 100% around a center frequency of 5 MHz, and the front surface or flaw echo is often several times larger than other echoes from within the metal. Because of multiple frequency components contained in broad-band pulses, digitization of such signals requires a high sampling rate to satisfy the Nyquist sampling criterion and a large word length to minimize the quantization error.
Obviously, a larger dynamic range or a smaller word length results in a larger quantization error in the digitized data. Thus, one way to decrease the quantization error is to use an A/D converter having larger word length. However, A/D converters having large word lengths are generally limited in speed. The cost of A/D converters significantly increases as the speed and word length thereof is increased. Of course, there is a limit as to the word length and the speed which can be achieved with conventional A/D converters. As this limit is approached, the cost of the A/D converter greatly increases. Thus, there is a need for some other means of minimizing the quantization error of conventional A/D converters without increasing the word length thereof.